Cannot get variable.replace working properly.

Question

Hello,

I am trying to replace a string with a new string in a python file and write the new string permanently to it. When I run the below script it removes part of the string and not all of it. The string in the file is:

self.id = "027FC8EBC2D1"

And the script I have to replace the string is:

def edit():

    o = open("test.py","r+") #open 
    for line in open("test.py"):   
        line = line.replace("027FC8EBC2D1","NewValue")  
        o.write(line) 
    o.close()

edit()

Thanks for any help.

Answer 1

You're opening the file as read-only and trying to write to it. And you've also got it open twice at once.

You'll want to reorganize it so that you've only got it open once, and that it is read-write.

Answer 2

Try something like this:

import fileinput
for line in fileinput.input('test.py', inplace=1):
  line.replace("027FC8EBC2D1","NewValue")

That makes your call to fh.close() extraneous (the with handles it) and prevents you from having multiple copies of the file open at a time.

Answer 3

You cannot safely do what you intend to do, unless the replacement value and the original value have exactly the same length. Unless this is guaranteed, I'd copy the file:

with open('input.txt', 'r') as in_file:
    with open('output.txt', 'w') as out_file:
        for line in in_file:
           line = line.replace('027FC8EBC2D1', 'NewValue')
           out_file.write(line)

EDIT (removed misleading information)

Answer 4

The proper way to do (actually simulate;-) "in-place substitution" on text files with Python is the fileinput module:

import fileinput

for line in fileinput.input(['test.py'], inplace=True):
  print line.replace('027FC8EBC2D1', 'NewValue'),

Note a couple of crucial details wrt the other answer that suggests the same module: input's first argument must be a list of filenames (not a string!), and, you do have to print every line that you want in the resulting file (fileinput redirects standard output to perform -- actually simulate -- the "overwrite in-place" effect).

A final small but not-negligible detail: the comma at the end of the print statement is to avoid adding another newline at the end (since each line already ends with a newline!-).

Answer 5

If you've got enough memory, compared to the size of your file, you can actually get away with this:

# Open the file for read/write access
f = open( 'test.py', 'r+' )

# Read the entire contents of the file into memory
data = f.read()

# Replace the old value with the new one
data.replace( '027FC8EBC2D1', 'NewValue' )

# Truncate the file (in case NewValue is shorter than OldValue)
f.truncate( 0 )

# Write all the data out again
f.write( data )

# Close the file
f.close()

I wouldn't recommend it for very large files, but it's going to be a faster solution than you might otherwise expect.